Add execution constraints to coverage.

[AlejandroCabeza]

Make codecov only run on PRs that target master. There are some PRs whose upload to codecov is silently failing. After a brief investigation, I think this is due to the free tier 250 monthly upload limit. Sticking to using codecov only in PRs that target master might be enough to stay under the limit.

Alternatively (or in addition), these two actions could be very handy:

• https://github.com/marketplace/actions/go-beautiful-html-coverage
• https://github.com/marketplace/actions/code-coverage-report-difference (if grcov could export to the go coverage format, or convert it some other way)